Toll switching telephone system having automatic amplifier insertion and gain setting



Jan. 1, 1952 Filed NOV. 14, 1947 IIG Bol. Net.

J. WICKS TOLL SWITCI'IING TELEPHONE SYSTEM HAVING AUTOMATIC AMPLIFIER INSERTION AND GAIN SETTING CENTRAL OFFICE 2O FIG.

Toll Boord 6 Sheets-SheeI l Jan. 1, 1952 J, WlCKS 2,580,709

TOLL SWITCHING TELEPHONE SYSTEM HAVING AUTOMATIC AMPLIFIER INSERTION AND GAIN SETTING Atys J. wlcKs 2,580,709 G TELEPHONE SYSTEM HAVING AUTOMATIC Jan. 1, 1952 TOLL SWITCHIN AMPLIFIER INSERTION AND GAIN SETTING 6 Sheets-Sheet 5 Filed NOV. 14, 1947 JNVENToR. John Wicks J. wlcKs 2,580,709 TOLL SWITCHING TELEPHONE SYSTEM HAVING AUTOMATIC Jan. 1 1952 AMPLIFIER INSERTION AND GAIN SETTING Filed NOV. 14, 194'? 6 Sheets-Sheet 4.`

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JNVENTOR. John Wicks Jan. l, 1952 J, W}KS 2,580,709

TOLL SWITCHING TELEPHONE SYSTEM HAVING AUTOMATIC AMPLIFIER INSERTION AND GAIN SETTING Filed Nov. 14, 194'? 6 Sheets-Sheet 5 John Wicks BY ma wnq/ww( Ays.

J. wlcKs 2,580,709 TOLL SWITCHING TELEPHONE SYSTEM HAVING AUTOMATIC Jan. 1, 1952 AMPLIFIER INSERTION AND GAIN SETTING 6 Sheets-Sheet 6 Filed Nov. 14, 194'? Patentedv Jan. l, 1952 UNITED *STATES 2or Fics Application November 14, rsirlgfs'eril n5786123 `7 claims. (01.419418) SAV'hepresent 'finventi'onf relates `to 1 tollv switching telephone system'sand more-.particularly to such I'lines and-trunks-'involvedl in--theftoll connections.

It lis anfl obj ect'fof Athelpresentinvention to' projvideWan improvedtolll switching vtelephone' system in which the toll'-connectionsaref first set' vup by automatic switching apparatus :and thereafter 'ampliiiersi are I inserted into the -f tollconnections `-in order =t0 establish ythe 'required I signal intensities over the"-'variou`siines Landi-trunks involved therein.

Another object ofthe-inventionis to provide fan*im"proved tollj switch-ingr telephone v`systemin which fthe nunrher ifemployed for setting 'automatic*switchinglfapparatusialso sets the required signal-gain fanassociatedamplifer'appropriate 'to the two sections of -theconnection lthat are joined by the switching-apparatus.

vAnother object of the invention-*isi to provide `aniinproved-4V combination automatic switch and amplier.

A"Afurtherobject of the invention isf to-pro'vide in l an autorriati-c `switch,- an improved v'a'.rra'gement @for inserting Lan amplifier incorporated therein between the incoming trunk land the switchingniechanism thereof. y

fAjfurtherfobjec't of'ithe inventionis to *provide vin an iautomatic*switch, an fim'proved arrangement'iion:selectively-'setting the`swit hingI mecha- Ynism"therefan'dfor selectivelysetting'a variable signal f 'gain f amplifier incorporated therein.

"A j'still further object ofthe invention is to jprovidean Zautomatic toll' 'switching telephone taken in @connection with `.the accompanying :drawings 'invlwhichFiguresvl to 6, inclusive, -take'n Atogether,ffconstitute a schematic diagram of the ,lai-'a served'lbyfautelephone'-system; embodying the Lpreser'it invention f and illustrates the Ydetails of 'certain-'cf the' apparatus incorporated in two dif- V-irfeit iiices A-of thelt'elephone system, which' ap- Sparatilshas -fincorporated' therein; the features fof the invention'f as briey: outlined above; -and Fig. 'lliiiustrates: the inode -of `combining Figs. 1 .tovk 6, inclllsiile,v toforinfa unified diagram.

fit/tereIpar-ticii1cr1y,-Figs.si'andi4 Yiiiusirate the dtailstf one :of l the t"o11}secon'd vselectors. and arnpliiierscorporatediinithevdistantf oice; land Figs'iarrdiillnstrate the details of one of the liiicm `g ltoilfse-letors landa1npliers incorporated? he'cen'traloiiice.

Referring nowlmoapaitcularly to'Figs. 1` to l6, `incli1sive,lof fthedraivings; the telephone system there illustrated-Serves lan `area comprising 'fa 'fdist ffice fijacentralfoce i205 and a -remote ce' 0. "The ldistant'oiice I- f is of the autov'-natic type,sewingY` arimber lof subscribr sbstations, includin'gV the? subscriber substation -Tl IJ, 'and-'om-priseslfa-toll board-360' ofthe manualdial ltype, afs welllas automatic switching apparatus.

iiiiarly, fthe-:central iiice 420 -is of the :auto- -inatic type, lservinglfa tvnii-Inter ofi subscriberv'vsbsystem of' theaudiofrequency dialing typewherein"the"'toll operators in the various exchanges of the system complete thevarious toll connections A`ntilizing "directory numbers and "wherein 'ampli'- er's "and V"k'ial'ancing 'networks `are "automatically "selected "and adjusted incident 'to J"operation of "the automatic switches incorporated in vthe'v 'system.

Further 'features 'of the invention 'pertain lto the fpaiticular 'arrangement lof the circuit 4Velev1:5 mentsof the system; whereby'theabove-outlined 'the :distant cnice ic and central office-2c' are as well as a toll switch train, including a group of incoming toll selectors and amplifiers 4|0, etc., individually associated with the repeaters 405, etc.; the incoming toll selector and amplifier 4|0 being individually connected to the repeater 405 by an outgoing trunk 401. Also the repeaters 405, etc., are provided with individually associated tone units 406, etc.; and the incoming toll selectors and amplifiers 4|0, etc., are provided with individually associated balancing networks 4H, etc. Also the toll switch train comprises a plurality of groups of outgoing toll selectors, one of the groups including the outgoing toll selector 310. The outgoing toll selectors 310, etc., are accessible via trunks 4|2, etc., to the incoming toll selectors and amplifiers 4|0, etc., and are provided with individually associated balancing networks 4|5, etc., also accessible via trunks 4|3, etc., to the incoming toll selectors and amplifiers 4|0, etc. Further the toll switch train comprises a plurality of groups of toll connectors, one of the groups including the toll connector 312. The toll connectors 312, etc., are accessible via trunks 31|, etc., to the outgoing toll selectors 310, etc.; and, in turn, have access to the subscriber lines 313, etc., respectively extending to the subscriber substations T|0, etc., in the associated group. Also the toll switch train comprises a group of toll trunk circuits 315, etc. The toll trunk circuits 315, etc., are accessible via trunks 314, etc., to the outgoing toll selectors 310, etc., and are connected via trunks 316, etc.. terminating jacks, not shown, disposed at the toll board 300. Further the toll switch train comprises a group of toll first selectors 303, etc., provided with individually associated toll line circuits 302, etc., terminating trunks 30|, etc., that terminate in jacks, not shown, disposed at the toll board 300. Further the toll switch train comprises a group of toll second selectors and amplifiers 400, etc., provided with individually associated balancing networks 418, etc., and accessible via trunks 320, etc., to the toll first selectors 303, etc. The toll second selectors and amplifiers 400, etc., have access via incoming trunks 420, etc., to the repeaters 405, etc. Also the repeaters 405, etc., are provided with individually associated balancing networks 435, etc., accessible via trunks 430. etc., to the toll second selectors and amplifiers 400, etc.

The central office 20 comprises a local switch train, not shown, for setting up connections to and from the subscriber substations T20, etc.; as well as a toll switch train, including first and second groups of incoming toll selectors and amplifiers respectively associated with the first and second groups of repeaters. The first group of incoming toll selectors and amplifiers includes the incoming toll selector and amplifier 600 individually associated with the repeater 500 in the first group; the incoming toll selector and amplifier 600 being individually connected to the repeater 500 by an outgoing trunk 5|0. The second group of incoming toll selectors and amplifiers includes the incoming toll selector and amplifier |53 individually associated with the repeater |50 in the second group; the incoming toll selector and amplifier |53 being individually connected to the repeater |50 by an outgoing trunk |52. Also the repeaters 500, etc., in the first group are provided with individually associated tone units 50| etc.; and the repeaters |50, etc., in the second group are provided with individually associated tone units |5I, etc. The incoming toll selectors and amplifiers 600, etc., in the first group are provided with individually associated balancing 4 networks 618, etc., and the incoming toll selectors and amplifiers |53, etc., in the second group are provided with individually associated balancing networks |54, etc. Also the toll switch train comprises a plurality of groups of outgoing toll selectors, one of the groups including the outgoing toll 'selector |10. The outgoing toll selector |10, etc., are accessible via trunks |60, etc.,

to the incoming toll selectors and amplifiers 600, etc., in the first group and to the incoming toll selectors and amplifiers |53, etc., in the second group, and are provided with individually associated balancing networks SI5, etc., also accessible via trunks 6I0, etc., to the incoming toll selectors and amplifiers 600, etc., in the first group and to the incoming toll selectors and ampliers |53, etc., in the second group. Further, the toll switch train comprises a plurality of groups of toll connectors, one of the groups including the toll connector |12. The toll connectors |12, etc., are accessible via trunks |1|, etc., to the outgoing toll selectors |10, etc., and, in turn, have access to the subscriber line |13, etc., respectively extending to the subscriber substations T20, etc., in the associated group. Also the toll switch train comprises a group of toll trunk circuits |15, etc. The toll trunk circuits |15, etc., are accessible via trunks |14, etc., to the outgoing vtoll selectors |10, etc., and are connected via trunks |16, etc., to terminating jacks, not shown, disposed at the toll board |00. Further the toll switch train comprises a group of toll first selectors |03, etc., provided with individually associated toll line circuits |02, etc., terminating trunks l0l, etc., that terminate in jacks, not shown, disposed at the toll board |00. Further the toll switch train comprises a first group of toll second selectors and amplifiers |05, etc., provided with individually associated balancing networks |06, etc., and accessible via trunks |04, etc., to the toll first selectors |03, etc. The toll second selectors and amplifiers |05, etc., have access via trunks ||0, etc., to the repeaters 500, etc., in the first group. Also the repeaters 500, etc., are provided with individually associated balancing networks |25, etc., accessible via trunks |20, etc., to the toll second selectors and amplifiers |05, etc., in the first group. Further, the toll switch train comprises a second group of toll second selectors and amplifiers ||5, etc., provided with individually associated balancing networks ||6, etc., and accessible via trunks ||4, etc., to the toll first selectors |02, etc. The toll second selectors and amplifiers H5, etc., have access via incoming trunks |30, etc., to the repeaters 50, etc., in the second group. Also the repeaters |50, etc., are provided with individually associated balancing networks |45, etc., accessible via trunks |40, etc., to the toll second selectors and amplifiers |l5, etc., in the second group. Moreover, the incoming toll selectors and amplifiers 600, etc., in the first group have access via the incoming trunks |30, etc., to the repeaters |50, etc., in the second group and to the balancing networks |45, etc., via the trunks |40, etc. Similarly, the incoming toll selectors and amplifiers |53, etc., in the second group have access via the trunks ||0, etc., to the repeaters 500, etc., in the first group and to the balancing networks |25, etc., via the trunks |20, etc.

The remote office 30 comprises a local switch train, not shown, for setting up connections to and from the subscriber substatins T30, etc.; as well as a toll switch train, including a group of incoming toll selectors and amplifiers 253, etc., individually associated with the repeaters 25|,

testent etc.,theincomingitoll selectorfand ampliiiery 253 `being individually-'connectedto the repeater 250 by'an utgoing'trunk 252. 'Also therrepeaters 250. etc., 'are vprovided 'with individually asso'- ciated tone units etc.; and the incoming toll selectors and ampliers 253, '-etc., are provided with individually associated balancing networks 254,-Tetc. Also the toll switch train Vcomprisesia plurality of groups -of'outgoing toll selectors, one of thegroups including the outgoing toll selector 210. The outgoing toll selectors 210, etc., are accessible via trunks 260,etc., to theincoming toll yselectors and amplifiers 253, etc., vand are provided with individually associated networks 285, etc.,`alsoacces`sible'via trunks 280,'etc., to the incoming toll-selectors andampliers 253, etc. V'Further-thetoll switch train comprises'a plurality of groupsff 'toll connectors, one of v'the groups including 'the toll-connector 212. The toll connectors'212, etc.,-areacces'sible via trunks 21|, etc., tof the outgoing t'olllselectors 210, etc.; and, in turn, havev access to the subscriber lines 213, etc., respectivelyextending to thesubscriber substations T30, etc.,v in the associated group. Also the toll switch train-comprises a group of toll trunk circuits 215, etc. Thetoll' trunk circuits`215,fetc., are'accessible via trunks 214, etc., to the voutgoing toll selectors 210, etc., andare connected via trunks 215,y etc., to terminating jacks, not shown, disposedatthe toll board 200. Further, the toll switch train comprises a group of toll 'rst selectors'203, etc., "provided .with individuallyassociated tollline circuits 20'2, etc., 4terminating tru'nks=20|, etc., that terminate in jacks, not shown, disposedat .the toll'board 200.

`networks 225, etc., accessible via trunks 220, etc.,

lto the toll -second selectors and ampliiiers 205, etc.

Sect; 2,.-The1- apparatus incorporated in 'the telephone system `In theseveral ofces, the'subscriber substa-v tions T|0,`T20, T30, etc.areprovided with conventionalsubstation apparatus; the subscriber lines 313, |13, 213, etc., are provided with'conventional line circuits, not shown, andthe local switchv trains, not shown, are of the conventional Strowger type. f Also the toll'boards 300,100 and 200=arefof the conventional. manual dial type, including cord' circuits,l not shown; the toll line circuits 302, |02-, 202, etc., are of any conventional ftype; the toll first selectors 303, |03, 203 etc., of the conventional Strowger type; the toll ,trunk circuits 315, |15, 215, etc., are of` any conventional type; the outgoing toll selectors 310, |10, 2210, etc., areof the conventional Strowger type; .and `the Ltollyconnectors 312, |12, 212 etc., ,are

of the conventional Strowger type.

The toll second selectorscand amplifiers 400, |05, H5, 205, etc., are identical; thetoll Asecond vselector` and ampliiier 40|)A comprises, as shown relay" H455. 'Also thelt'oll 'second'jselector'and y., 'amplifier'w comprises a Stro'wg'er'A mechanism 48u-*having six individual wipersltodinclusive, provided withan associatedcoritact bank, a vertical magnet M380 for Ydriving-the wiper cari riage step by step in the verticaldirection-away from its normal vertical positioma rotary magnet vM38| for driving1 the -wiper carriage -Ivstep?y by step in the rotarydirection away from itsnorm'al rotary position and aA release magnet M3831flfor releasing the wiper carriage-andfor-'returning-it to its normal rotary and vertical positions. Also associated with the'wiper carriage of lthe AStrow'ger mechanism 430- are three sets of switchsp'ring's S313, S314 and S315 that are operatedv when the wiper carriage is driven one step rin-Athe-ver-'tie cal direction away from its normal vertical position, and two sets of switch'springs S316and S311 that are operated when the vwwiper `carriage"isis driven elevensteps in the vrotary*directionfJawa'y from its normal rotary position. `-Furtherfthe toll second selectorand amplifier`4ll0J comprises a repeating coil `1190, two transformers-'Mlland 092, and two one-way amplifiers 493 andf494ff the vacuum tube type.

The incoming toll selectors 'and' amplifiers-W0, 000,- I53, 253, etc., are identical; thelincoming'toll selector andamplier comprises, asshown in- Figs. 5 and 6, a relay group including' a switch relay R530, a line relay R540-, a hold-relay R550-, aA transfer relay R550, aprepare relay R630, a'--switch through relay Rtlftwo control relays R650-2and R555, a series relay Rlfand aring"-relay^fR665. Also the incoming toll selector-and-ampl-ierl comprises a Strowger mechanismA 680f'havingsix individual wipers 58| to 580, inclusive,'provided with-aiiassociated contact bank, av vertical magnet M500 for driving the vwipericarriage step? by step in the vertical'direction'away from its-normalvertical position, a rotary -magnetMalfor driving the wiper carriage step by stepfin' the lrotary drection'away from its normalrotaryfpc'sition and a release magnet'M583 forieleasing'tl-i'e wiper'carriage and for returning it to its normal rotary and vertical positions. `Also lassociated with the wiper carriage ofthe' StrowgerY mechanism 500 are three sets of switch ,springs-S511 S515 and S515 that are operated when the-"wiper carriage is `driven one step in the verticaldi'r'ec'- tion away from its'normal vertical positionfarfid two sets of switchv springs S516-andS511- ,that are operated when ythe-wiper carriageisdriven-eleven steps in the rotarydirection awa-y' from itsno'rm'al rotary position. FurtherA the Vincoming A'jtll 'se lector and amplier'-i500` comprises a-repeating coil 530, two transformers-09| and692,iand---two one-way ampliers 003 `andfSdfof "the vacuum tube type. v

Further the incoming toll selectorJ and ampli"- er 500 comprises a 'gain-setting' vswitchrl'ZOof the minor type; including two individualwip'e'rs 02| and E22 provided 'with a`ssociated'contact banks, a rotary magnet -lVl62-3-fcr^drivingthe wipers noted step by step in the counterclockwise direction, and a release magnet M624"for"reieas ing the wipers noted and` for returningithemo 'their normalu'otary positions. Also-fassoitd with the wipersv 02| and 022Y of the'ga'insetting switch 020is aset'of switch lsprings S625"`tht"ji operated when the wipers', noted are'drivenvon'e stepin the counterclockwise direction away. rom their normal rotary positions. The repeaters its, soc, est, 25e, etcgareieem tical and ofthe connection andarrangementoif the repeater 400 disclosed in the copen'din'gjappl'ilcation of JohnWi'cks and Otho D,Grand'sta`i,

serieu No. '149.758, ieuMayzz; iwf/,newratem 7 No. 2,517,516, granted August 1, 1950. Specifically, the repeater 400 disclosed in Figs. 4, 5 and 6 of the Wicks and Grandstai application mentioned comprises a relay network.

The tone units 406, 50|, |5|, 25|, etc., are identical and of the connection and arrangement of the tone unit disclosed in the copending application of Otho D. Grandstaff, Serial No. 738,194, filed March 29, 1947. Specifically, the tone unit |00 disclosed in the single figure of the Grandstaff application mentioned comprises a relay and vacuum tube network, and is capable both of generating and of receiving 60 cycle modulated 1000 cycle current and unmodulated' 1000 cycle current, and also of amplifying ordinary audio signals received over the associated toll line and for impressing the amplified audio signals on the associated incoming trunk for monitoring purposes.

The voice current repeaters 505A, 505B, |80A, |80B, etc., incorporated in the toll lines 505, |80, etc., are of any suitable vacuum tube type. The balancing networks 4| I, 618, |54, 254, etc.. individually associated with the incoming toll selectors and amplifiers 4|0, 600, |53, 253, etc., are of conventional form; the balancing networks 418, |06, ||6, 206, etc., individually associated with the toll second selectors and amplifiers 400, |05, ||5 205, etc., are of conventional form; and the balancing networks 4|5, 435, 6|5, |45, |25, 225, and 285, accessible to the various toll second selectors'- and amplifiers 400, etc., and to the various incoming toll selectors and amplifiers 4 0, etc., are of conventional form. l

More'v particularly, the balancing network 4|| is individually associated with the incoming tollV selector and amplifier 4|0 and is arranged tov balance the toll line 505 via the outgoing trunk 401 and the terminating repeater 405; likewise the balancing network 618 is individually'associated with the incoming toll selector and amplifier 500 and is arranged to balance the toll line 50 5 via the outgoing trunk 5|0 and the terminating repeater 500. Similarly, the balancing network |54 is individually associated with the incoming toll selector and amplifier |53 and is arranged to balance the toll line |80 via the outgoing trunk |52 and the terminating repeater |50; likewise, the balancing network 254 is individually associated with the incoming toll selector and amplifler 253 and is arranged to balance the toll line |80 via the outgoing trunk 252 and the Lterminating repeater 250.

The' balancing network 435 is individually pairedl with the terminating repeater 405 and .'ranged to balance the toll line |80.

is selected via the trunk 430 when the repeater 405 is` selected via the incoming trunk 420 by the toll second selector and amplifier 400, etc., and is arranged to balance the toll line 505; likewise, the balancing network |25 is individually paired with the terminating repeater 500 andl is selected via the trunk |20 when the repeater 5001s selected via the incoming trunk I0 by the toll second selector and amplifier |05, etc., in the', first group and by the incoming toll selector and amplifier |53, etc., in the second group, and is arranged to balance the toll line 505. Similarly, the balancing network 225 1s individually paired with the terminating repeater 250 and'is selected via the trunk 220 when the repeater 250 is selected via the incoming trunk 2|0 by the toll second selector and amplifier 50, etc., and is arranged to balance the toll line |80; likewise, the balancing network |45 is individually paired with the terminating repeater |50 and is selected The balancing network 4|5 is paired with the outgoing toll selector 310 and is selected via the trunk 4|3 when the outgoing toll selector 310 is selected via the incoming trunk 4|2 by the `incoming toll selector and amplifier 4|0, etc., and is arranged to balance a local connection in the distant office |0 either to a subscriber substation Tl0, etc., or to the toll board 300 therein. Likewise, the balancing network 6|5 is paired with the outgoing toll selector |10 and is selected via the trunk 6|0 when the outgoing toll selector |10 is selected via the trunk |60 by the incoming toll selector and amplifier 600, |53, etc., and is arranged to balance a local connection in the central oflice 20 either to a subscriber substation T20, etc., or to the toll board |00 therein. Similarly, the balancing network 285 is paired with the outgoing toll selector 210 and is selected via the trunk 280 when the outgoing toll selector 210 is selected via the incoming trunk 260 by the incoming toll selector and amplifier 253, etc., and is arranged to balance a local connection in the remote office 30 either to a subscriber substation T30, etc., or to the toll board 200 therein.

The toll second selectors and amplifiers 400, |05, etc., have fixed gain settings therein to accommodate the attenuations of the local connections in the oflices |0 and 20 and of the toll lines 505, etc., extending between the-'offices mentioned. likewise, the toll second selectors and amplifiers 205, ||5, etc., have fixed gain settings therein to accommodate the attenuations of local connections in the offices 30 and 20 and of the toll lines |80, etc., extending between the oices mentioned. The incoming toll selectors and amplifiers 600, etc., comprise a gain setting therein to accommodate the attenuation of the toll line 505, etc., and first and second selectable gain settings respectively corresponding to the attenuation of a local connection to a subscriber substation T20, etc., or to the toll board |00 in the central ofllce 20 and to the attenuation of the toll lines |80, etc., extending between the central office 20 and the remote office 30; likewise, the incoming toll selectors and amplifiers |53, etc., comprise a gain setting therein to accommodate the attenuation of the toll lines |80..

etc., and first and second selectible gain settings respectively corresponding to the attenuation of a local connection to a subscriber substation T20, etc., or to the toll board |00 in the central ofiice 20 and to the attenuation of the tolljlines 505, etc., extending between the distant ofiice I0 and the central ofilce 20.

A better` understanding of the connection and arrangement of the apparatus incorporated in the telephone system will be facilitated from a. consideration of the extension of calls involving the various offices thereof.

Sect. 3.-Extenson of a call from the toll board in the distant o17ce to a subscriber substation in the central oice Assuming that a toll call is to be extended from the toll board 300 in the distant ofilce I0 to a subscriber substation, such for example, as the subscriber substation T20 in the central ofiice :atentos 20, the 4operator at the toll board 300 selects an idl cord circuit and inserts the front plug kthereof intovthe jack terminating an idle one of the trunks in the group, including the trunk 351i. For example, the trunk 30| may be thus selected; whereby the associated toll line circuit 302 operates to effect seizure of the associated toll first selector 303. .The operator at the toll board 300 then dials a predetermined iir'st digit inorder to elect operation of the toll first selector 303; whereby the toll rst selector V303 selects the corresponding group of trunks, including the trunk 320, and then an idle trunk in the selected group. For example, the toll first-selector 333 may select the particular trunk 320 extending to the toll second selector and amplifier 400, the trunk 320 being marked as ridle to the toll iirst selector 303 by the application of battery potential to the control conductor C323 thereof,V via the set of switch springs S315-and the upper winding of the transfer relay R350. When the toll first selector 303 selects the idle trunk 320. it switches through whereby ground potential therein is 'applied to the control wiper 32 thereof and consequently to the control conductor 0323er the trunk 320 in order to complete a circuit substantially identical to that previously traced for operating the transfer relay R360. Also the application of ground potential upon the control conductor C323 of vthe trunk 320 completes a circuit, includingthe contacts 333 :and 33t, and the line conductors C322 kand 032| of the trunk 323, for energizing in series the upper and lower windings of the line relay R343 in order tocause the latter 'relay to operate. Upon operating the `line relay R340 completes, at the-contacts 34|, ra circuit, including the contacts 331, for operat- -ing the hold relay R350. Upon 'operating the hold relay R350 completes, at the contacts 35i, a direct path for applying ground Vpotential to the control conductor C323 of the trunk 3210, thereby to complete a multiple holding-,circuit for energizing the 'upper' winding of the transfer relay R360 and a 4direct yloop lcircuit for energizing in series the upper and lower windings of the line relay R340 from the toll hoard 300 via the connection, including the line conductors C32i and C322 of the trunk 320, the toll riirstselector 303, the toll line circuit 302 and the `trunk v33%. Also upon operating the hold vrelay R353 completes, at the contacts 352, a circuit forenergizing the intermediate winding of the 'switch relay R330 Vin order to render the latter relay fast to operate when the 4lower winding thereof yis subsequently energized. Further the fholdrelay R353 prepares, at the contacts 353, a point in a circuit traced hereinafter for `energizing the lower winding of the switch relay R330; and prepares, at the contacts 354, a point in a multiple circuit traced vhereinafter for 'energizing the llower winding of the transfer relay R360 and `for operating the vertical magnet M380. At this time the .toll 'first selector 333 has operated toseize the trunk 320 extending to the toll second :selector .and amplier `400 and the tollsecond selectorand amplifier 403 is conditioned Eto be responsive vto the-'seconddigitdialed at the toll-board'333. Also 'the application of Vground `potential tothe control conductor C323 of the-trunkf-320 marks :the ltrunk 320 as busy to the other toll rstselectors yhaving access thereto.

The "operator at `the toll "board v300 thenproceeds to dial a'-predeterrninedfseconddigit; whereby the line `relay R340 follows over the loop circuit. Each time the line relayR340 .restores It) and then reoperates, it interrupts, and then recompletes, at the contacts 34H, the circuit for energizing the winding of the hold relay R350; whereby the latter relay, being of the slow-torelease type, remains operated during impulsing.

vAlso each time the line relay R343 restores and then reoperates, it completes and then interrupts, at the contacts 342, a circuit, including the contacts 331, 354 and 303, for energizing the lower winding of the transfer relay R330 in order to retain the latter relay in its operated position during impulsing, as it is of the slow-to-release type. Further, cach time the line relay R340 restores and then reoperates, it completes and then interrupts, at the contacts 342, a multiple circuit, including the contacts 331 and 354, for operating the vertical magnet M330; whereby the wiper carriage of the Strowger An'iechanisxn'llilil is driven a corresponding number ofsteps in the vertical directionv away from its Ynormal vertical position. When the wiper carriage of the Strowger mechanism 480 is driven one step in the vertical direction away from .its normal vertical position, the `sets of switch springs S313, S314and S315 are operated. More particularly the set of switch springs S315 is operated in order to interrupt the circuit for energizing the upper winding of the transfer relay R350; and the set of switch springs S313 is operated in order to prepare a circuit traced hereinafterfor loperating the rotary magnet M38I.

At the conclusion of the second digit inentioried, the line relay R340 is vretained in its oper.- ated position and shortly thereafter, Vthe transfer relay R333 restores, `it being of .the slow-toreleasetype. Also at this time the wiper carriage vof the kStrowger mechanism 48.3l has selected the group of trunks, including the trunk `423 extending to the repeater 435 terminating the toll line 535 extending between the distant oiice l0 and the central Aoilice 23. r Upon restoring the transfer relay R360 completes, at the contacts 334, a path, including the contacts 331 and 343 and the set of switch .springs S313, for operating the supervisory relay R310. Upon operating vthe supervisory relay R313 completes, vat the contacts 31 I a path for applying battery po,- tential by way of the `resistor 312' to lthe EC con.- ductor C324 of the trunk .3.2.3, thereby to illuminate the front supervisory lamp associated with the cord circuit in use v at the toll board k33,0 in order to .indicate @to the `operator ther-eat ,that the Vthird digitshould not -be `dialed at this ,time since the toll second selector and amplifier 409 is proceeding with -the selection oi an idle 70911 line extending between the distant office 'i0 .and the central oilice :20. Also `upon restoring the transfer relay R333 completes, atvthe .contacts 334, a multiple circuit, .including the .contacts 3311, 343 `and 382, and the fset of Vswitch springs S313, for operating the rotary Amagnet M381;

whereby the rotary magnet M381 upon operating interruptsfat vthe contacts 382, the operating circuit therefor and `drives the wipers ,noted .of the 'Strowger mechanism A4,30 one ,-step-,lrthe rotary direction away Afrom their normal rotary positions. At this time the switch relay R333 `tests lthe idle or vlousycondi-tion of the 'rst trunk .in -the group, including Athe 4trunk 423, ground potential appearing .upon the r.control conductor thereof in the event the selected trunkmentoned is busy completing la path, vvincluding the ,control wiper 335 of the Strowger mechanism 483, the contacts 332, 353 yari`d35l, for short-circuitin'g the iewerwinding of tneswitch relay Rast. es..

suming that the selected rst trunk mentioned is busy at this time, the rotary magnet M384 then restores in order to recomplete, at the contacts 382, the operating circuit thereof; whereby the rota-ry magnet M38| reoperates in order to interrupt, at the contacts 382, the operating circuitv therefor; whereby the rotary magnet M38| reoperates in order again to interrupt, at the contacts 382, the operating circuit therefor and to drive the wiper carriage of the Strowger mechanism 480 an additional step in the rotary direction selecting the second trunk in the group, including the trunk 420. Accordingly, the rotary magnet M38| operates buzzer fashion; whereby the Wiper carriage of the Strowger mechanism 480 is driven step by step in the rotary direction and the corresponding selected trunks are tested.

Assuming that the trunk 420 is the iirst idle trunk in the selected group, when it is selected by the wiper set of the Strowger mechanism 480, battery potential appearing upon the control conductor C423 thereof completes a circuit substantially identical to that previously traced `for energizing the lower winding of the switch relay R330 in order to cause the latter relay to operate exceedingly fast, it being polarized as a consequence of the previous energization of the intermediate winding thereof. Upon operating the switch relay R330 interrupts, at the contacts 331,

the multiple circuits for operating the rotary I magnet M38| and for energizing the windings of the hold relay R350 and the supervisory relay R310, thereby to arrest further operation of the rotary magnet M38l and to cause the hold relay R350 and the supervisory relay R310 to restore l shortly thereafter, the relays mentioned being of the slow-to-release type. vAlso the switch relay R330 completes, at the contacts 335, a holding circuit, including the grounded control conductor C323 of the trunk 320, for energizing the upper winding thereof; and completes, at the contacts 338, a direct path, including the contacts 352, for applying ground potential to the control wiper 485 of the Strowger mechanism 480 and consequently to the control conductor C423 of the` trunk 420 extending to the repeater 405. Further the switch relay R330 interrupts, at the contacts 33|, and 333, the loop circuit extending from the toll board 300 to the upper and lower windings of the line relay R340; and completes, at the contacts 332 and 334, a connection, including the con'- tacts 443, 445 and 44|', 44.9 between the line conductors C32| and C322 of the trunk 320 and the line wipers 4.83 and 484 of the Strowger mecha'- nism 480; which connection includes the winding of the series relay R460 and is extended via the line conductors C42| and C422 of the trunk 420 to the line relay in the repeater 405. Accordingly, at this time the toll second selector and amplifier 400 has operated to seize the trunk 420 extending to the repeater 405; whereby the loop circuit from the toll board 300 is extended via the trunk 30|, the toll line circuit 302, the toll rst selector 303, the trunk 320, the toll second selector and amplifier 400 and the trunk 420 to the line relay in the repeater 405. Accordingly, the repeater 405 is seized and operates in conjunction with the associated tone unit 406 in a manner more fully explained hereinafter. When the switch relay R330 interrupts, at the contacts 33| and 333, the original loop circuit extending to the upper and lower windings of the line relay R340, the latter relay restores. Upon restoring the hold relay R350 interrupts, at the contacts 35|, the previously mentioned path for applying l2 ground potential in the toll second selector and amplier 400 to the control conductor C323 of the trunk 320; however, without effect, since ground potential in the toll rst selector 333 is retained upon the associated control wiper 3|2 and consequently upon the control conductor C323 of the trunk 320 in order to retain operated the switch relay R330 in the toll second selector' and amplifier 400. Also upon restoring the hold relay R350 interrupts, at the contacts 353, the original operating circuit for energizing the lower winding of the switch relay R330; and interrupts, at the contacts 352, the circuit for energizing the intermediate winding of the switch relay R330; however, without effect, since the holding circuit for energizing the upper winding of the vswitch relay R330 is completed at this time as previously noted. Upon restoring the supervisory relay R310 interrupts, at the contacts 31|', the original path for applying battery potential by way of the resistor 312 to the EC conductor C324 of the trunk 320; however, upon operating the switch relay R330 completed, at the contacts 338, a connection between the EC conductor C324 of the trunk 320 and the EC Wiper 486 of the Strowger mechanism 480 and consequently the EC conductor C424 of the trunk 420; whereby battery potential in the repeater 405 is applied to the EC conductor C424 of the trunk 420 in order to retain illuminated the front supervisory lamp associated with the cord circuit in use at the toll board 300, thereby to continue the caution to the operator at the toll board 300 not to dial the third digit at this time. Also the operated switch relay R330 retaining ground potential upon the control wiper 485 of 'the Strowger mechanism 480 and consequently upon the control conductor C423 of the trunk 420, marks the trunk 420 as busy to the other toll second selectors and amplifiers having access thereto. As previously noted, the trunk 430 extending to the balancing network 435 is paired with the incoming trunk 420 extending to the re peater 405; whereby the wipers 48| and 482 of the Strowger mechanism 48D- select the line conductors C43| and C432 of the trunk 430 when the wipers 483 and 484 of the Strowger mechanism 480 select the line conductors C42| and C422 of the trunk 420; the balancing network 435 being arranged to balance the toll line 535 extending between the distant oice I0 and the central oflice 20. Also it is noted that the winding of the series relay R460 in the toll second selector and amplifier 400 is energized over the loop circuit extending between the toll board 300 and the line relay in the repeater 405; whereby the series relay R460 operates. Upon operating the series relay R460 completes, at the contacts 45|, a circuit for operating the control relay R455; and upon operating the control relay R455 completes, at the contacts 451, a circuit for operating the control relay R450.

When the toll second selector and amplier 430 is operated in the manner previously explained to seize the trunk 420 extending to the repeater 405, the repeater 405 operates in conjunction with the associated tone unit 406 to transmit a short splash of cycle modulated 1000 cycle current over the toll line 505, and constituting a seizure signal; and also to return during the transmission of the seizure signal battery potential over the EC conductor C424 of the trunk 420. At the conclusion of the seizure signal the repeater 405 and the associated tone unit 406 are conditioned to be receptive to a digit transmitted from cuit therefor.

`or.` busy condition of the first trunk mentioned and assuming that it is busy, ground Potential appearing upon the control conductor thereof completes a path, including the control wiper 665 of the Strowger mechanism 680, the contacts '562, 553, and 55|, for short-circuiting the lower .winding of the switch relay R530. Accordingly, ,f the rotary magnet M58| restores in order to recomplete, at the contacts 582, the operating cir- Upon reoperating 'the rotary magnet M58| again interrupts, at the contacts 582, the operating circuit therefor and drives the .wiper carriage of the Strowger mechanism 680 an additional step in the rotary direction. Accordingly, the rotary magnet M58| operates buzzer fashion as the trunks inthe associated group are tested as busy by the switch relay R530.

`Assuming that the trunk |60 is the first idle .trunk in the selected group, when it is selected by the wiper set of the Strowger mechanism 680,

batteryr potential appearing upon the control conductor C|63 thereof completes a circuit substantially identical to that previously traced for energizing the lower winding of the switch relay R530; whereby the latter relay operates exceed. 4ingly fast, it being polarized by virtue of the completed circuit for energizing the intermediate winding thereof. Upon operating the switch relay R530 interrupts, at the contacts 531, the circuit for operating the rotary magnet M58l; and

. completes, at the contacts 535, a holding circuitl including the grounded hold conductor C5|3 of the trunk 5|0 for energizing the upper winding thereof. Also the switch relay R530 completes, -at the contacts 536, a direct path, including the vcontacts 562, for applying ground potential to the control wiper 685 of the Strowger mechanism 660: and consequently to the control conductor C|63yof the trunk |60 whereby the trunk- |60 .extending to the outgoing toll selector is 'marked as busy to the other incoming toll selectors and ampliers having access thereto. gAlso the switch relay R560 interrupts, at the contacts 531, a circuit-for energizing the winding of the hold relay R550 in order to cause the Alatter relay to restore shortly thereafter, it, being of the slow-to-release type. Further the switch relay R530 interrupts, at the contacts 53| and 533,

the loop circuit extending between the repeater 500 and the upper and lower windings of the line'relay-R540 in order to cause the latter relay to restore. Further the switch relay R530 cornpletes, at the contacts 532 and 534, a connection including the contacts 643', 645 and 64|', 649 between the line conductors 4C5| |A and C512 of the trunk 5|0 and the line wipers 683 and 684 of the.

Strowger mechanism' 680 and consequently to the line conductors C|6| and C| 62 of the trunk '|60'. This connection includes'the winding of 'the series relay R660; whereby a loop circuit is thereof. Upon restoring the hold relay`R550 interrupts, at thecontacts 55|, the path for applyving ground potential in the incoming toll selector'and amplifier 600 to the control conductor C5|3 of the trunk 5|0; however, ground potential in the repeater 500 is retained upon the con- -p-trol conductor C5|3 of the trunk 5|0 in order to retain operated the switch relay R530. Also upon Operaties the Switeh. `relay-R531) completes. at.,

16 the contacts 538, the connection between the EC conductor C5|4 of the trunk 5|0 and the EC wiper 686 of the Strowger mechanism 680 and conse- -quently the'EC conductor C|64 of the trunk |60 extending to the outgoing toll selector |10. Also upon restoring the hold relay R550 interrupts. at the contacts 552, the circuit for energizing the intermediate winding of the switch relay R530; and interrupts, at the contacts 553, the original operating circuit for energizing the lower winding oi' the switch relay R530; however without effect at this time. Upon operating the series relay R660 completes, at the contacts 66|, a circuit for operating the control relay R655; and upon operating the controlrelay R655 completes, at the contacts 651, a circuit for operating the control relay R650. l

At this time the incoming toll selector and amplifier 600 has responded to the third digit to select the correspinding group of trunks, includ- A.ing the trunk |60, and has operated automatii network 6|5 is paired with the trunk 60 extending to the outgoing toll selector |10; whereby the wipers 68| and 682 of the Strowger mechanism 680 select the line conductors C6|| and C6|2 of the trunk 6| 0 when the wipers 683 and 684 of the Strowger mechanism 680 select the line conductors C|6| and C|62 of the trunk |60; the balancing network 6|5 being arranged to balance a local connection in the central oflice 20 extended eitherto one of the subscriber substations T20, etc., or to thetoll board |00 therein.

As previously noted, the third digit dialed at the tollboard 300 in the distant oftlce 0 effected operation vof the Strowger mechanism 680 to select the group of trunks including the trunk and also the digit was registered in the gain setting switch 620. More particularly, the wiper 62| of the gain setting switch 620 engages a contact in the associated contact bank corresponding to vthe thirdA digit, thereby to select a corresponding one of the resistors 626 and to connect the selected resistor 626 in multiple with the fifth or driving winding of the transvformer 69|; whereby the effective gain of the amplier 694 is established appropriate to the attenuation of the toll -line 505. Similarly, the wiper 622 4ofv the gain setting switch 620 engages a contact v in the associated contact 'bankcorresponding to the third digit, thereby -to selecta corresponding one of the resistors 621 and to connect the selected resistor 621 in multiple with the fifth ordriving winding of the transformer 692; whereby the effective ,gain of the amplier 693 is established appropriate to a local connection either to one of the subscriber substations T20, etc., or to the toll board |00 in the central oiilce 20.

'I'he operator at the toll board 300 proceeds to dial the fourth digit mentioned; whereby the ,line relay in the repeater 405 follows causing the repeater 405 to cooperatewith the associated tone runit 406 vto transmit a corresponding series of splashes of unmodulated 1000 cycle current over I the toll line 505, and constituting a dialing signal.

The repeater 500 and the associated tone unit 50| respond to the dialing signal and repeat a corresponding series of impulses, constituting the fourth digit, over the loop circuitl extending via the trunk |0, the incoming toll selector and amplier 600 and the trunk |60 to the outgoing toll selector whereby the outgoing toll selector |10 is operated to select the group of trunks, including the trunk I1 I, and then an idle trunk in the selected group. Assuming that the outgoing toll selector selects the particular trunk |1| extending to the toll connector |12, the outgoing toll selector |10 switches through to extend the loop circuit via the trunk i1| to the toll connector |12; whereby the toll connector |12 is conditioned to be responsive to the fth and sixth digits dialed at the toll board 300 in the distant office |0.

The operator at the toll board 366 proceeds to dial the fth and sixth digits mentioned; whereby the repeater 405 responds cooperating with the associated tone unit 406 to transmit two corresponding dialing signals over the toll line 505 as previously explained; whereby the repeater 505 and the associated tone unit 50| respondin order to transmit two corresponding series of impulses over the loop circuit extending to the toll connector |12; whereby the toll connector |12 is operated to select the subscriber line |13 extending to the called subscriber substation T in the central oiiice 20. The subsequent operation of the toll connector |12 depends upon the idle or busy condition of the called subscriber substation T at this time, the subscriber line |13 being marked as idle or busy by the associated line circuit, not shown.

First assuming that the called subscriber substation T20 is idle at this time, the toll connector |12 operates to seize the subscriber line |13 and to dry or interrupt the supply of battery current to the loop circuit extending back via the trunk |1|, the outgoing toll selector |10, the trunk |60, the incoming toll selector and amplifier 600 and the trunk 5|0 to the repeater 500. When this loop circuit is thus dried, the winding of the series relay R660 in the incoming toll selector and amplifier 600 is deenergized; whereby the latter relay restores to interrupt, at the contacts 66|, the circuit for energizing the winding of the control relay R655; whereby the latter relay restores shortly thereafter, it being of the slow-to-release type. Upon restoring the control relay R655 interrupts, at the contacts 651, the circuit for energizing the winding of the control relay R650; whereby the latter relay restores shortly thereafter, it being of the slow-to-release type. Also upon restoring the control relay R655 completes, at the contacts 656, a circuit, including the contacts 65|, for operating the switch-through relay R640. Upon operating the switch-through relay R640 completes, at the contacts 64|, a holding circuit, including the contacts 562 and 536, for energizing the winding thereof. Also the switchthrough relay R640 interrupts, at the contacts 643, 645, 649 and 64|', the direct connection between the line conductors C5|| and C5|2 of the trunk 5||i and the line wipers 633 and 548 of the Strowger mechanism 680; completes, at the contacts 642 and 644, a connection, including the contacts 532 and 534, between the line conductors C5|| and C5|2 of the trunk 5|6v and the first and second windings of the repeating coil 660; completes, at the contacts 646 and 641, a connection between the conductors C616 and C611 of the trunk 615 extending to the balancing network 616 and the fourth and second windings of the transformer 69|; completes, at the contacts 646 and 640, connections between the line wipers 684 and 683 of the Strowger mechanism 660 and the first and third windings of the trans- 18 former 692; and completes, at the contacts 642 and 643', connections between the wipers 682 and 68| of the Strowger mechanism 660 and the fourth and second windings of the transformer 692. Accordingly, at this time the trunk 5|0 is operatively connected to the rst and second windings of the repeating coil 690; the third and fourth windings of the repeating coil 690 are connected Via the rst, third and second, fourth windings of the transformer 69| to the balancing network 618; the balancing network 618 being arranged to balance the toll line 505 as previously noted. Moreover, the balancing network 6|5 is connected Via the second, fourth and first, third windings of the transformer 692 to the trunk 60 and consequently via the outgoing toll selectorY |10, the trunk |1|, the toll connector |12 and the subscriber line |13 to the called subscriber substation T20; the balancing network 6|5 being arranged to balance the local connection to the called subscriber substation T20 as previously explained. -Also when the toll connector |12 switches through it applies ground potential to the EC conductor of the trunk |1| and consequently Via the connection to the EC conductor C|64 of the trunk |60 and consequently Via the EC wiper 606 of the Strowger mechanism 660 and the contacts 533 to the EC conductor C5|4 of the trunk 5|0 extending from the repeater 500. Accordingly, when the loop circuit extending back over the line conductors C5|| and C5|2 of the trunk 5|0 is dried and ground potential is applied to the EC conductor C5|4 of the trunk 5|0, the repeater 500 is controlled to transmit a splash of battery potential over the positive line conductor C5 2 of the trunk 5|0, thereby to complete a circuit, including the contacts 534 and 644 and the second winding of the repeating coil 690, for operating the ring relay R665. Thus, the ring relay R665 operates and then restores completing and then interrupting, at the contacts 661, a path, including the contacts 643, for applying a splash of battery potential Via the resistors 668 and the line wiper 684 of the Strowger mechanism 680 to the positive line conductor C162 of the trunk |60 and consequently over the connection to the positive line conductor of the trunk |1| extending to the toll connector |12; whereby the toll connector |12 is operated to institute a ringing cycle. More particularly, the toll connector |12 operates to project ringing current over the subscriber line |13 to the called subscriber substation T20 in order to operate the ringer thereat indicating the presence of a call waiting to be answered.

Also the repeater 500 is controlled in conjunction with the tone unit 50| at this time to send a splash of 60 cycle modulated 1000 cycle current over the toll line 505 and constituting anv endof-dialing signal. rIhe repeater 405 and the associated tone unit 466 respond to the end-of-dialing signal to dry or interrupt the supply of battery current to the loop circuit extending from the line relay therein back to the toll board 300; whereby the winding of the series relay R460 in the toll second selector and amplifier 400 is interrupted causing the latter relay to restore. Also the repeater 405 operates to return ground potential over the EC conductor C424 of the trunk 420 and consequently over the EC conductor C324 of the trunk 320 thereby to illuminate the front supervisory lamp associated with the cord circuit in use at the toll board 300 in order to indicate to the operator thereat that the connection has been extended to the called subscriber substation T20 in the central ofce 20 and that the called 19 subscriber substation T20 is being rung at this time. Further, the repeater 405 operates in conjunction with the tone unit 405 to complete a monitoring connection between the line conductors of the toll line 505 and the line conductors C42| and C422 of the trunk 420; whereby the end-of-dialing signal onAthe line conductors of the toll line 595 is amplified and impressed upon the line conductors C42| and C422 of the trunk 420 so that it may be heard as an audible signal at the toll board 300. Upon restoring the series relay R400 interrupts, at the contacts 46|, the circuit for energizing the winding of the control relay R455 in order to cause the latter relay to restore shortly thereafter, it being of the slowto-release type. Upon restoring the control relay R455 interrupts, at the contacts 451, the circuit for energizing the winding of the control relay R450; whereby the latter relay restores shortly thereafter, it being of the slow-to-release type. Also upon restoring the control relay R455 completes, at the contacts 455, a circuit, including the contacts 45|, for operating the switch-through relay R440. Upon operating the switch-through relay R449 completes, at the contacts 44|, a holding circuit, including the contacts 302 and 335, for energizing the winding thereof. Also the switchthrough relay R440 interrupts, at the contacts 443, 445, and 44|', the direct connection between the line conductors C32| and C322 of the trunk 320 and the line wipers 483 and 484 of the Strowger mechanism 490; completes, at the contacts 442 and 444, a connection, including the contacts 332 and 334, between the line conductors C32| and C322 of the trunk 320 and the rst and second windings of the repeating coil 49D; completes, at the contacts 44S and 441, a connection between the conductors C410 and C411 of the trunk 15 extending to the balancing network 418 and the fourth and second windings of the transformer 49|; completes, at the contacts 448 and 440, connections between the line wipers 484 and 483 of the Strowger mechanism 480 and the rst and third windings of the transformer 492; and completes, at the contacts 442 and 443', connections between the wipers 40?. and 48| of the Strowger mechanism 480 and the fourth and second windings of the transformer 492. Accordingly, at this time the trunk 320 is operatively connected to the first and second windings of the repeating coil 490; the third and fourth windings of the repeating coil 490 are connected via the first, third and second, fourth windings of the transformer 49| to the balancing network 410; the balancing network 418 being arranged to balance the local connection back to the toll board 300, as previously noted. Moreover, the balancing network 435 is connected via the second, fourth and first, third windings of the transformer 492 to the trunk 420 and consequently:

to the repeater 405; the balancing network 435 being arranged to balance the toll line 505, as previously explained.

When the subscriber at the called subscriber substation T20 in the central oice 29 Vanswers' 509 operates in conjunction with the tone'unit"v 50| to send a short splash of 60 cycle modulated 1000 cycle current over the toll line 505, and constituting an answer signal. The repeater 405 and the associated tone unit 49S respond to the answer signal; whereby the repeater 405 switches through interrupting the monitoring connection between the line conductors of the toll line 595 and the line conductors C42| and C422 of the trunk 429 and removing ground potential from the EC conductor C424 or the trunk 429. When ground potential is thus removed from the EC conductor C424 of the trunk 429 and consequently from the EC conductor C324 of the trunk 320, the front supervisory lamp associated with the cord circuit in use at the toll board 309 is eX- tinguished in order to indicate to the operator thereat that the call has been answered at the called subscriber substation T29 in the central oflice 20.

At this time an established connection is completed between the calling toll board 300 in the distant office |0 and the called subscriber substation T20 in the central oiiice 20. Specifically, the operators headset at the toll board 300 is connected via the trunk 30|, the toll line circuit 302, the toll rst selector 303 and the line conductors of the trunk 320 to the first and second windings of the repeating coil 490 in the toll second selector and amplifier 400. The third and fourth windings of the repeating coil 490 are connected via the rst, third and second, fourth windings of the transformer 49| to the balancing network 418; the balancing network 435 is connected via the second, fourth and iirst, third windings of the transformer 492 to the line conductors of the trunk 420 and thence to the repeater 405. The repeater 4'95 is operatively connected via the toll line 505 incorporating the voice current repeaters 505A and 505B to the repeater 500. The repeater 500 is connected to the line conductors of the trunk 5|0 that, in turn, are connected to the first and second windings of the repeating coil 690. The third and fourth windings of the repeating coil 690 are connected via the first, third and second, fourth windings of the transformer 69| to the balancing network 618; and the balancing network 6|5 is connected via the second, fourth and first, third windings of the transformer 592 to the line conductors of the trunk |60 and thence via the outgoing toll selector |10, the trunk |1|, the toll connector |12 and the subscriber line |13 to the called subscriber substation T20 in the central ofce 20. Also in the toll second selector and amplifier 400, the junctions between the rst,'second and third, fourth windings of the transformer'49l are connected to the input circuit of the amplifier 493; and the output circuit of the amplifier 493 is connected to the fth or driving winding of the transformer 492; similarly, the junctions between iirst, second and third, fourth windings of the transformer 492 are connected to the input circuit of the amplifier 494; and the output circuit of the amplifier 494 is connected to the fifth or driving winding of the transformer 49|. Likewise, in the incoming toll selector and amplifier 009, the junctions between the first, second and third, fourthwind ings of the transformer 69| are connected to the input circuit of the amplifier 693; and the 'output circuit of the amplifier 693 is connected to the fifth or driving winding of the transformer 692; similarly, the junctions between the rst, second and third, fourth windings of the transformer 692 are connected to the input circuit of 2l the amplifier 694; and the output circuit of the amplifier 694 is connected to the fifth or driving winding of the transformer 69|.

Accordingly, whenY the operator at the toll board 399 speaks, the audio currents traversing the trunk 39|, the line circuit 392, the toll first selector 393 and the trunk 329 traverse the rst and second windings of the repeating coil 4'99 inducing corresponding audio currents in the third and fourth windings thereof; which induced audio currents traverse the first, third and second, fourth windings of the transformer 49| and the balancing network 418. The voltage drop across the second and fourth windings of the transformer 49| and the balancing network 418 is applied to the input circuit of the amplifier 493; whereby the amplified audio currents in the output circuit of the amplifier 493 traverse the fifth or driving winding of the transformer 492. The audio currents traversing the fifth Winding of the transformer 492 induce algebraically additive audio voltages in the first and third windings thereof that are applied to the line conductors rof the trunk 429 extending to the repeater 495; and also induce conjugate audio voltages in the second and fourth windings thereof which are productive of no audio potential applied to the input circuit of the amplifier 494, thereby preventing singing of the amplifiers 493 and 494'. The audio currents traversing the repeater 495 cause corresponding audio currents to traverse the toll line 595 incorporating the voice current repeaters 595A and 595B and conseg quently the repeater 599. The audio currents traversing the repeater 599 cause corresponding audio currents to traverse the trunk 5|9 and the first and second windings of the repeating coil 699. The audio currents traversing the first and second windings of the repeating coil 699 induce corresponding audio currents in the third and fourth windings thereof; which induced audio currents traverse the first, third and second, fourth windings of the transformer 69| and the balancing network 618. The voltage drop across the second and fourth windings of the transformer 69| and the balancing network 618 is applied to the inputV circuit of the amplifier 693; whereby the amplified audio currents in the output circuit of the amplifier 693 traverse the fifth or driving winding of the transformer 692, as well as the multiple connected selected resistor 621. The audio currents traversing the fifth Winding of the transformer 692 induce algebraically additive audio voltages in the first and third windings thereof that are applied to the line conductors of the trunk |69 extending to the outgoing toll selector |19; and also induce conjugate audio voltages in the second and fourth windings thereof which are productive of no audio potential applied to the input circuit of the amplifier 694, thereby preventing singing of the amplifiers 693 and 694. The audio currents traverse the toll outgoing selector |19, the trunk |1|, the toll connector |12, the subscriber line |13 and the substation equipment at the called subscriber substation T29. i

Similarly, when the subscriber at the called subscriber substation T29 speaks, the audio currents traversing the subscriber line |13, the toll connector |12, the trunk I1 I, the outgoing toll selector |19 and the trunk |69 traverse the first, third and second, fourth windings of the transformer 692 and the balancing network 9 5. The voltage drop across the second and fourth windings of the transformer 692 and the balancing network 6| 5 is applied to the input circuit of the amplifier 694; whereby the amplified audio currents in the output of the amplifier 694 traverse the fifth or driving winding of the transformer 69 l, as well as the multiple connected selected resistor 626.- The audio currents traversing the fth winding of the transformer 69| induce algebraically additive audio voltages in the first and third windings thereof that traverse the third and fourth windings of the repeating coil 699; and also induce conjugate audio voltages in the second and fourth windings thereof which are productive of no audio potential applied to the input circuit of the amplifier 693, thereby preventing singing of the amplifiers 693 and 694. The audio currents traversing the third and fourth windings of the repeating coil 699 induce corresponding audio currents in the first and second windings thereof which traverse the trunk 5H! and the repeater 599. The audio currents traversing the repeater 599 cause corresponding audio currents to traverse the toll line 595 incorporating the voice current repeaters 595B and 595A and consequently the repeater 495. The audio currents traversing the repeater 599 cause corresponding audio currents to traverse the trunk 4.29 and the first, third and second, fourth windings of the transformer 492 and the balancing network 435. The voltage drop across the second and fourth windings of the transformer 492 and the balancing network 435 is applied to the input circuit of the amplifier 494; whereby the amplified audio currents in the output of the amplifier 494 traverse the fifth or driving winding of the transformer 49|. The audio currents traversing the fifth winding of the transformer' 49| induce algebraically additive audio voltages in the first and third windings thereof that traverse the third and fourth windings of the repeating coil 499; and also induce conjugate audio voltages in the second and fourth windings thereof which are productive of no audio potential applied to the input circuit of the amplifier 493, thereby preventing singing of the amplifiers 493 and 494. The audio currents traversing the third and fourth windings of the repeating coil 499 induce corresponding audio currents in the first and second windings thereof which traverse the trunk 329, the toll first selector 693, the toll line circuit 392 and the trunk 39| and the headset at the toll board 399.

In this toll connection, the selected resistor 621 connected in multiple to the fifth or driving winding of the transformer 692 insures the required effective gain of the amplifier 693 in the connection extending to the called subscriber substation T29; while the selected resistor 626 connected in multiple to the fifth or driving winding of the transformer 69| insures the required effective gain of the amplifier 694 in the connection extending via the toll line 595 to the calling toll board 399.V Similarly,` in this toll connection the coupling between the fifth or driving winding of the transformer 49| and the rst and third windings thereof, is selected to insure the required effective gain of the amplifier 494 in the connection extending to the calling toll board 399; while the coupling between the fifth or driving winding of the transformer 492 and the first and third windings thereof is selected to insure the required effective gain of the amplifier 493 in the connection extending via the toll line 595 to the called subscriber substation T29.

At the conclusion of the established connection, when the subscriber at the called subscriber substation .T disconnects, the toll connector .H2

Yresponds to apply 60 ground impulses perminute to the EC conductor of the trunk .11| and consequently to the EC conductor C|64 of the trunk |60 and the EC conductorC5|4 of the trunk 5| 0; whereby the repeater 500 and the associated tone unit 50| respond to send 60 splashes per minute of 60 cycle modulated l000.cycle current over the toll line 505, and constituting a disconnect signal. The repeater 405 and the associated tone unit 406 respond to the disconnect signal to apply 60 ground impulses per minute to the EC conductor C424 of the trunk 420 and consequently to the EC conductor C324 of the trunk 320; whereby the front supervisory lamp associated with the cord circuit in use at the toll board 300 is flashed 60 times per'minute, and constituting a disconnect signal to the operator thereat.

Upon receiving the disconnect signal, the operator at the toll board 300 may disconnect to eiect the release of the connection or re-ring the subscriber at the called subscriber substation T2 0. First assuming that the operator re-rings the subscriber at the called subscriber substation T20, this is accomplished by momentary operation of the ring key at the toll board 300; Whereby a splash of battery potential is connected to the positive line conductors of the trunk and consequently to the positive line conductor C322 of the trunk 320 completing a circuit, including the contacts 334 and 444 and the second winding of the repeating coil 400, for operating the ring relay R465. Accordingly, the ring relay R465 operates and then restores, completing and then interrupting, at the contacts 461, a path, including the contacts 448 and the line wiper 484 of the Strowger mechanism 480, for applying battery potential Via the resistor 468 to the positive line conductor C422 of the trunk 420 extending to the repeater 405. The repeater 405 and the associated tone unit 406 respond and send a short splash of 60 cycle modulated 1000 cycle current over the toll line 505, and constituting a re-ring signal. The repeater 500 and the associated tone unit respond to the re-ring signal to apply a splash of battery potential to the positive line conductor C5|2 of the trunk 5|0 completing a circuit, including the contacts 534 and 644 and the second winding of the repeating coil 690, for

operating the ring relay R665. Accordingly, the ring relay R665 operates and then restores completing and then interrupting, at the contacts 661, a path, including the contacts 648 and the line wiper 684 of the Strowger mechanism 630,

for applying a splash of battery potential via the resistor 668 to the positive line conductor C|62 of the trunk and through the connection to the positive line conductor of the trunk |1| extending to the toll connector |12. The toll connector |12 responds to the splash of battery potential upon the positive line conductor of the trunk |1| to initiate another ringing cycle thereof. More particularly, the toll connector |12 operates to project ringing current over the subscriber line |13 in order to operate the ringer at the called subscriber substation T20 in order again to signal the subscriber thereat.

When the subscriber at the called subscriber substation T20 reanswers the call, the toll connector |12 operates to interrupt the application of 60 ground impulses per minutes from the EC conductor of the trunk |1| and consequently from the EC conductor C|64 of the trunk |60 and the EC conductor C5|4 of the trunk 5|0 effecting the release of the connection. particularly, the operator at the vtoll board 300 .extendingzto the .repeater 500. Therepeater 500 and the associated tone unit 50| .respond .to arrest transmission of thedisconnect signal .over

.the toll line 505. lTheirepeater 405 and the associated tone unit 406respond to the `termination ofthe disconnect signal over the toll line 505 to interrupt the application of 60 ground impulses per minute from the EC conductor C424 of the trunk 420 and consequently from the EC conductor C324 of the trunk 320 in order again toextinguish the front supervisory lamp associated with the cordcircuit in use at the toll board 300 in order again to indicate to the operator thereat that the call has again been answered at the called subscriber substation T20.

When the operator at the toll board 300 receives the disconnect signal she may disconnect More withdraws the front plug of the cord circuit in use from the jack terminating the trunk 30| in order to effect the release of the tollline circuit 302 and the toll first selector 303. When the toll iirst selector 303 is thus released, ground potential therein is removed from the control conductor C323 of the trunk 320 interrupting the .holding-circuit for energizing the upper winding of the switch relay R330 in order to cause the latter relay to restore. Upon restoring the switch relay R330 interrupts, at the contacts 336, the holding circuit for retaining operated the switchthrough relay R440 and the multiple path for applying ground potential via the control Wiper V485 of the Strowger mechanism 480 to the control conductor'C423 of the trunk 420 extending to the repeater 405. Also upon restoring the switch relay R330 completes, at the contacts 331, a circuit, including the contacts 342 and 355 and the set of switch springs S314, for operating the release magnet M383; whereby the wiper carriage of the Strowger mechanism 480 is released and returned to its normal rotary and Vertical positions. When the wiper carriage of the Strowger mechanism is thus released, the sets of switch springs S313, S314 and S315 are operated. More particularly, the set of switch springs S314 is operated to interrupt the operating circuit for the release magnet M383; and the set of switch springs S315 is operated in order again to apply battery potential by way of the upper winding of the transfer relay R360 to the control conductor C323 of the trunk 320, again marking the trunk 320 and the toll second selector and amplier 400 as idle tothe toll first selectors 303, etc., having access thereto.

The repeater 405 and the associated tone unit 405 respond to the removal of ground potential from the control conductor C423 of the trunk 420 to send a long splash of 60 cycle modulated 1000 cycle current'over the toll line 505, and constituting a release signal; and then the repeater 405 and the associated'tone unit 400 are released; whereby battery potential in the repeater 405 is again applied to the control conductor C423 of the trunk 420 again marking the trunk 420 and the repeater 405 as idle to the toll second selectors and amplifiers 400, etc., having access thereto. The repeater 500 and the associated tone unit 50| respond to the release signal transmitted over the toll line 505 to interrupt the application of ground potential to the control conductor C5|3 of the trunk 5| 0, thereby to interrupt the holding circuit for energizing the upper winding of the switch relay R530 in order to cause the latter relay to restore. The repeater 500 and the as.

aaaoyroc sociated tone unit 50| are then released, the repeater 500 operating to apply battery potential to the control conductor of the incoming trunk in order again to mark the incoming trunk ||0 and the repeater 500 as idle to the toll second selectors and amplifiers |05, etc., and to the incoming toll selectors and amplifiers |53, etc., having access thereto.

Upon restoring the switch relay R530 interrupts, at the contacts 536, the holding circuit for retaining operated the switch-through relay R640 and the multiple path for applying ground potential by way of the control wiper 685 of the Strowger mechanism 600 tothe control conductor C|63 of the trunk |60. Also upon restoring the switch relay R530 completes, at the contacts 531, a circuit including the contacts 542 and 555 and the set of switch springs S514, for operating in multiple the release magnets M583 and M624. Upon operating the release magnet M503 releases the wiper carriage of the Strowger mechanism 680 causing it to be returned to its normal rotary and vertical positions; whereupon the sets of switch springs S513, S514 and S515 are operated. Upon operating the release magnet M624 releases the wipers 62| and 622 of the gain setting switch 620 causing them to be returned to their normal rotary positions; whereupon the set of switch springs S625 is operated. More particularly, the set of switch springs S514 is operated to interrupt the multiple operating circuits for the release magnets M583 and M624; and the set of switch springs S515 is operated in order again to apply battery potential by way of the upper Winding of the transfer relay R560 to the control` conductor C|3 of the trunk 5|0. When ground potential is removed from the control conductor C|63 of the trunk |60, the outgoing toll selector and consequently the toll connector |12 are released. Battery potential in the outgoing toll selector |10 is returned upon the control conductor C|63 of the trunk |60 in order to mark the trunk |60 and the outgoing toll selector |10 as idle to the incoming toll selectors and amplifiers 600, etc., having access thereto. Similarly, battery potential in the toll connector |12 is returned upon the control conductor of the trunk |1| in order to mark the trunk |1| and the toll connector |12 as idle to the outgoing toll selectors |10, etc., having access thereto. Also battery potential in the line circuit, not shown, associated with the subscriber line l 'i3 is returned upon the control conductor thereof in order again to mark the subscriber line |13 extending to the subscriber substation T as idle to the toll connectors |12, etc., and to the connectors in the local switch train, not shown, having access thereto. At this time the established connection between the calling toll board 300 in the distant oice I0 and the called subscriber substation T20 in the central oflice 20 is released and all of the apparatus involved therein is completely released and available for further use.

Reconsidering the extension of the connection from the calling toll board 300 in the distant office l0 to the called subscriber substation T20 in the central office 20, it may occur that the called subscriber substation T20 is busy when the toll connector |42 operates in response to the sixth digitl to select the subscriber line |13 extending thereto. In this case, the toll connector |l2 operates in order to dry or interrupt the supply of battery current to the loop circuit extending back over the connection tothe repeater 500 and to apply 60 ground impulses per 26 minute to the EC conductor of the trunk |1| and consequently to the ECfconductor C|64 of the trunk |60 and the EC conductor C5|4 of the trunk 5|0. When the loop circuit mentioned is thus dried, the series relay R660 restores effecting restoration of the control relays R655 and R650 and the consequent operation cf the switchthrough relay R640; all in the manner previously explained. Also the repeater 500 and the as' sociated tone unit 50| respond to send 60 splashes per minute of 60 cycle modulated 1000 cycle current over the toll line 505, and constituting a line busy signal. In passing, it is noted that the line busy signal is identical to the disconnect signal previously mentioned. The repeater 405 and the associated tone unit 406 respond to the line busy signal to dry or interrupt the supply of. battery current to the loop circuit extending back to the toll board 300; whereby the series relay R460 restores effecting the restoration of the control relays R455 and R450 and the consequent operation of the switch-through relay R440 in the manner previously explained. Also the repeater 405 operates to apply 60 ground impulses per minute to the EC conductor C424 of the trunk 420 and consequently to the EC conductor C324 of the trunk 320 in order to effect ashing 60 times per minute of the front supervisory lamp associated with the cord circuit in use at the toll board 300. The flashing 60 times per minute of the front supervisory lamp mentioned indicates to the operator at the toll board 300 that the connection has been extended to the called subscriber substation T20 and that the subscriber line extending thereto has been tested by the toll connector in use as being busy. Also the repeater 405 responds to complete the previously mentioned monitoring connection between the line conductors of the toll line 505 and the line conductors C42| and C422 of the trunk 420 via the amplifying arrangement in the associated tone unit 406; whereby the operator at the toll board 300 receives a cor-v responding audio line busy signal. The operator at the toll board 300 then releases the connection thus far extended by withdrawing the front plug of the cord circuit in use thereat from the jack terminating the trunk 30| in order to effect the release of the operated apparatus in the manner previously explained.

Also during the extension of the connection from the calling toll board 300 after the outgoing toll selector |10 responds to the fourth digit, it may test as busy all of the trunks in the group, including the trunk |1|; whereby the outgoing toll selector |10 operates to dry or interrupt the supply of battery current to the loop circuit extending back over the connection to the repeater 500 and to apply 120 ground impulses per minute to the EC conductor C|64 of the trunk |60 and' consequently to the EC conductor C5|4 of the trunk 5|0 extending to the repeater 500. When the loop circuit mentioned is thus dried, the incoming toll selector and amplifier 600 is switched through; and the repeater 500 and the associated tone unit 50| operate to send 120 splashes per minute of 60 cycle modulated 1000 cycle current over the toll line 505, and constituting a trunk busy signal. The repeater 405 and the associated tone unit 406 respond to the trunk busy signal to dry or interrupt the supply of battery current to the loop circuit extending back to the calling toll board 300 and to apply 120 ground impulses per minute to the EC conductor C424 of the trunk 420; whereby the toll second selector and amplifier 400 is switched through and the front superagee-0,7091

visoryv lamp associated with thecord circuiti'n. use at'thetoll board v300 is flashed 120 times. perv minute'indicating to the operator thereat that an all-trunks-busy condition has been encountered in the toll switch train. AlsoV the-repeater 405 responds to complete the previously mentionedv monitoring connection between the line conductors of'the toll line 505 and the line conductors C42| and C422 of the trunk 420 via the amplifying.

arrangement in the associated tone unit 406; whereby the operator at the toll board 300receives a corresponding audio trunk busy signal. The operator at the toll board 300then withdraws the frontplug of the cord circuit in use from the jack terminating the trunk 30| in order to bring about the release of the apparatus operated at this'time.

Also during the extension of the connection from the calling toll board 300 after the incomingtoll selector` and'amplier 'responds to the third'digit, it may testas busy all of the trunks in'..the group, including the trunk |60; whereby thev wiper carriage of the Strowger mechanism 680 is driven eleven'steps in the rotary direction away from itsnormal rotary positionI in order to `operate the sets of switch springs S516 and S511'. More particularly, the set of switch springs S516 is operated to complete a circuit for energizing in series the upper winding ofthetransfer relay R550 and the lower winding of the switch relay R530.` via` the grounded control conductor C|3 of the trunk 5|0; whereby the relays mentioned` operate. Upon operating the switch relay R530 effects the restoration of the line relay R540 and the consequent restoration of the hold relay R550k shortly thereafter, the latter relay being of the: slow-to-release type. Also upon operating, the switch relay R530 interrupts, at the contacts 531, the operating circuit for the rotary magnet M58| in order to arrest further operation of the Strowger mechanism 680 in the rotary' direction. Also the-.set of switch springs S511 is operated to connect ther 120 ground impulses per minute conductor C518 to the EC conductor C5|4 of the trunk 5|0".v Accordingly, at thistime the loop circuit from the repeater 500 is dried in that thel supply of battery current thereto is interrupted; whereby the repeater` 500 and the associated tone unit 50| operate to send 120 splashes'per minute' of 60 cycle modulated 1000 cycle current over the toll line 505, and constituting the trunk-busy signal. The repeater 405 and the associated tone unit 406 respond to the trunk-busy signal in order to bring about flashing of thefront supervisory lamp associated with thecord circuit in use at the -toll board 300 in the manner previouslyl explained, thereby rendering to the operator at the toll board 300 all-trunk-busy supervision. Also the repeater 405 respondsto lcomplete thepreviouslyv mentioned monitoring connection between the'linel conductors of the toll line 505 and theline conductors C42| and C422 of thetrunk 420 via the'amplifying arrangement inv theV associated tone unit 406; whereby the operator at the toll board 300' receives a corresponding` audio trunk busy signal. Illhe operator at the toll board 300 then withdraws the front plug of the cord circuit in use from the jack terminating the trunk 30| in order to bring about the lrelease of the apparatus operated at this time.

Similarly, during the extension of the connection from the calling toll board 300 after the toll second selector and amplier 400 responds to the second digit, it may test as busy all of the trunks inthe group, includingrthe trunk 420, whereby the wiper carriage of the Strowger mechanism 483 isI driven eleven steps in the rotary direction away from-,its normal rotaryk position in orderto operate the sets. of switch springs S316 and S311'. More particularly, the set of switch springs S315 is operated to complete av circuit for energizing in series the upper winding of the transfer relay R360 and the lower winding of the switch relay R330 via the grounded control conductor R323 of the trunk 320; whereby the relays mentioned operate. Upon operating the switch relay R330 effects the restoration of the line relay R340 and the consequent restoration of the hold relay R350 shortly thereafter,I the latter relay being of the slow-to-release type. Also upon operating, the switch relay R330 interrupts, at the contacts 331, the operating circuit for the 'rotary magnet M38| in order to arrest further operation of the Strowger mechanism 480 in the rotary direction. Also the set 'of switch springs S311 is operated to connect the" ground impulses per minute conductor C318 to the EC conductor C324 of the trunk 320; Accordingly, at this time the front supervisory lamp associated with the cord circuit in use at the toll board 300 is flashed 120 times per minute in order to indicate to the operator thereat that an all-trunksbusy condition has been encountered in the toll switch train. The operator at the toll board 300 then withdraws the front plug of the cord circuit in use from the jack terminating the trunk 30| in order to bring about the release of the apparatus operated at this time.

Finally, during the extension of the connection from the calling toll board 300', after the toll iirst selector 303 responds to the rst digit, it may test as busy all of the trunks in the group, including the trunk 320; whereby the toll first selector 303 operates to apply 120 ground impulses per minute through the toll line circuit 302 to the EC conductor of the trunk 30| in order to bring about flashing 120 times per minute of the front supervisory lamp associated with the cord circuit in use at the toll board 300 rendering to the operator thereat all-trunks-busy supervision. The operator at the toll board 300 then withdraws the iront plug of the cord circuit in use from the `iack terminating the trunk 30| in order to bring about the release of the apparatus operated at this time.

Sect. 4.-Etensicn of a call from the toll board in the distant olice to the toll board in the central oce The extension of a call from the toll board 300 in the distant oiiice I0 to the toll board |00 in the central office 20 is fundamentally the same as the extension of a call to the called subscriber substation T20 in the central oce 20 described in Sect. 3. In this' case, the operator at the toll board 300 may select 'the toll rst selector 303 via the toll line circuit 302 and the trunk 30E; the toll iirst selector 303 may respond to the first digit to select the trunk 320 extending to the toll second Selector and amplifier 400; the toll second selector and amplifier 400 may respond to the second digit to select the incoming trunk 420 extending to the repeater 405; the repeater 405 may seize over the toll line 505 the repeater 500; the repeater 505 may seize over the outgoing trunk 5 0 the incoming toll selector and amplier 600; the incoming toll selectorand amplifier 000 may respond to the third digit to select the trunk |60 extending to the outgoing toll selector |10; and the outgoing toll selector |10 may respond to the fourth digit to select rst the group of trunks, including thc ;trunk. |14 and thenanidle trunk therein. For 

